1. Field of the Invention
The present invention is related to a printed circuit board, and more particularly to an electromagnetic bandgap structure and a printed circuit board solving a mixed signal problem between an analog circuit and a digital circuit.
2. Background Art
With an increasing importance of mobility, various kinds of devices, including mobile communication terminals, PDAs (Personal Digital Assistants), notebook computers, and DMB (Digital Multimedia Broadcasting) devices, are being introduced in the market.
These kinds of devices include a printed circuit board that consists of an analog circuit, such as an RF circuit, and a digital circuit.
FIG. 1 is a cross-sectional view of a printed circuit board that consists of an analog circuit and a digital circuit.
A printed circuit board 100 includes metal layers 110-1, 110-2, 110-3 and 110-4 (hereinafter collectively referred to as 110), dielectric layers 120 (subdivided into 120-1, 120-2 and 120-3), which is layer-built between the metal layers 110, and a digital circuit 130, which is mounted on the top level metal layer 110-1, and an RF circuit 140.
Supposing that the metal layer represented by reference numeral 110-2 is a ground layer and the metal layer represented by reference numeral 110-3 is a power layer, an electric current flows through a via 160 connected between the ground layer 110-2 and the power layer 110-3, and the printed circuit board 100 serves a predetermined function or operation.
Here, an EM wave 150, caused by an operating frequency and harmonic components of the digital circuit 130, is transferred to the RF circuit 140, causing a mixed signal problem. The mixed signal problem means disturbing an accurate operation of the RF circuit 140 because the EM wave from the digital circuit 130 has a frequency within the operating frequency of the RF circuit 140. For example, when the RF circuit 140 receives a predetermined frequency signal, accurate receipt of the signal may be difficult because the EM wave including the predetermined frequency signal is transferred from the digital circuit 130.
It is becoming harder to solve this mixed signal problem because digital devices become more complex and the operating frequency of the digital circuit becomes higher.
Using a decoupling capacitor, which is a typical solution for the power noise problem, can not be a proper solution in high frequency, and studies for a structure of blocking a high frequency noise between the RF circuit and the digital circuit are needed.
FIG. 2 illustrates the cross section of an electromagnetic bandgap structure solving the mixed signal problem between an analog circuit and a digital circuit in accordance with the related art, and FIG. 3 illustrates a plan view of metal plates arrangement of the electromagnetic bandgap structure shown in FIG. 2. FIG. 4 illustrates a perspective view of the electromagnetic bandgap structure shown in FIG. 2, and FIG. 5. illustrates an equivalent circuit of the electromagnetic bandgap structure shown in FIG. 2.
An electromagnetic bandgap structure 200 includes a first metal layer 210-1, a second metal layer 210-2, a first dielectric layer 220a, a second dielectric layer 220b, a metal plate 232 and a via 234.
The first metal layer 210-1 and the second metal layer (210-2) are connected through the via 234 and the metal plate 232, and the via 234 forms a mushroom type structure 230 (refer to FIG. 4).
In case the first metal layer 210-1 is a ground layer, the second metal layer 210-2 is a power layer, and in case the first metal layer 210-1 is a power layer, the second metal layer 210-2 is a ground layer.
That is, by repetitively arranging the mushroom type structure 230 consisting of the metal plate 232 and the via 234 between the metal layer and power layer (refer to FIG. 3), a bandgap structure, which blocks a signal within a certain frequency range, is formed.
The function that blocks the signal within a certain frequency range is resulted from the resistance (RE, RP), inductance (LE, LP), capacitance (CE, CP, CG), conductance (GP, GE) components and can be represented as an equivalent circuit, as shown in FIG. 5.
A typical digital device in which a digital circuit and an RF circuit are implemented on the same board is the mobile communication terminal, which requires noise blocking between 0.8 and 2.0 GHz, which is the operating frequency range of the RF circuit. Moreover, the size of the mushroom type structure must be small enough to be used in the mobile communication terminal. In case of using the aforementioned electromagnetic bandgap structure, however, the two problems can not be solved at the same time.
As the mushroom type structure becomes smaller, the bandgap frequency in which the noise is blocked becomes higher, the noise blocking is little effective in the 0.8˜2.0 GHz range, which is the operating frequency of the RF circuit in the mobile communication terminal.